ORIGIN OF MYCETOCYTES IN PSEUDOCOCCUS. 293 



cleus is of course quite evident. In Psendococcus for instance, its 

 derivatives tend to stay at the periphery and do not sink into the 

 egg as does the pronucleus. Nevertheless those inherent quali- 

 ties in the latter which cause it to fuse with the male pronucleus, 

 may be present to a certain extent also in the polar nucleus deriv- 

 atives and cause them to combine with any cell that happens to 

 come in contact with them. Certainly this tendency is not to be 

 observed in the cleavage cells, for these are never found to fuse 

 with each other under ordinary circumstances. It is found only 

 in the polar nucleus derivatives, which can fuse both with each 

 other and with the cleavage cells. 



In much of their further behavior they are not anomalous at 

 all. It is almost unnecessary to mention that in case of a great 

 many animals and plants, complete fusion of the pronuclei may 

 be delayed for some time. In such extreme cases as Cyclops 

 (Riickert, '95; Haecker, '95) and Qryptobranchus (Smith, '19) the 

 individuality of the two pronuclei may be traced even through the 

 early cleavage stages. The failure of immediate fusion of two 

 apposed nuclei in Pseudococcus is therefore not peculiar. As a 

 matter of fact, it seems to be a rule in insects that the two pronu- 

 clei lie in apposition and the chromosomes of each are evolved 

 independently of those of the other. It is only when nearly fully 

 condensed that the nuclear walls break down and the chromo- 

 somes intermingle. Such seems to be the case in Archimerus 

 (Morrill, '10), "goumi aphid" (Stevens, '06), Trialeurodes 

 (Schrader, '20) and finally in the pronuclei of Pseudococcus 

 itself. 



Another aspect in the process of fusion of polar nucleus deri- 

 vatives and cleavage cells is to be observed in the fact that two 

 apposed nuclei may be in different phases. It has been explained 

 that at such times the chromosomes of the nucleus in a more 

 advanced phase are fully condensed but then enter on a period of 

 suspended activity. During this period the chromosomes of the 

 apposed nucleus or nuclei also become condensed and only then 

 the common spindle is formed and the different sets of chromo- 

 somes are arranged in a single plate. The mechanism involved in 

 this regulative process is not clear to me. But it may be stated 

 that this aspect also is parallelled by the behavior of pronuclei 



